Sinn Féin has been accused of using its six MPs to engage in "pointless" lobbying on Brexit at a time when they could potentially sway key Westminster votes in favour of Irish interests.

Labour leader Brendan Howlin has said he's "disappointed, but not surprised" that new Sinn Féin president Mary Lou McDonald has said she doesn't see the party changing its policy on abstentionism.

Ms McDonald was challenged on the issue by Newstalk's Pat Kenny. With the minority Conservative government relying on DUP support, votes on Brexit are likely to be tight.

But Ms McDonald said Sinn Féin was an "abstentionist party" and added: "I don't envisage us changing that stance."

She said Sinn Féin's MPs had been "consistently lobbying" politicians and "banging the drum for all of Ireland in respect of Brexit".

Put to her that Sinn Féin MPs could sway a vote on the final Brexit deal, she replied: "There is no prospect of any deal... which will render Brexit null and void."

Last night, Mr Howlin told the Irish Independent "lobbying is pointless" and added: "Parliament obviously boils down to a vote and the majority carries the day."

He said there was likely to be "a once-in-a-generation, critical vote for the people of Ireland" in Westminster on whether or not Britain remains in some form of customs union with the EU now that the UK Labour Party supports such a stance.

He said he had respected Sinn Féin's policy of abstentionism but that there had to be a point where the good of the people overrules that principle.

Mr Howlin accused Sinn Féin of putting "interests of the party before what's in the best interests of the people of Ireland".

A Sinn Féin spokesman responded by saying: "If the Labour Party wants Irish politicians to take their seats in Westminster, then they should stand candidates in the North on that basis and stop hurling from the ditch."

Separately, Ms McDonald was forced to defend her use of the phrase 'tiocfaidh ár lá" in the speech where she assumed her party leadership.

Mr Kenny suggested she had used the slogan to "give a nod to the hard men in Belfast".

Ms McDonald denied this and insisted: "The conflict has gone away, you know. The IRA has gone away, you know, and Sinn Féin is led now by a woman from the city of Dublin who has had no involvement in the conflict, with the IRA or anything else."